The carcinogenic and non-carcinogenic mechanisms of As are incompletely understood, but an emerging body of evidence suggests that As exposure leads to epigenetic dysregulation. In 4 independent studies in our Bangladesh cohort we have demonstrated that chronic As exposure is associated with increased global DNA methylation, contingent on adequate folate status. We hypothesize that the mechanism underlying this relates to As-induced alterations in histone modifications. Two potential mechanisms include a) in vitro, As induces G9a mRNA and protein expression; G9a is a central player in epigenetic regulation, and b) As is a very potent inhibitor of pyruvate dehydrogenase, an enzyme that catalyzes the final step in Acetyl CoA biosynthesis; inhibition of Acetyl CoA biosynthesis may decrease histone acetylation leading to chromatin condensation and increased DNA methylation. Folate is a key regulator of one-carbon metabolism mediated methylation reactions, including methylation of DNA and histones. A large randomized trial is currently underway in Bangladesh to evaluate the effects of folic acid (FA) supplementation on As methylation. We propose a cross-disciplinary collaboration that will take advantage of a unique repository of samples collected from this trial to carry out a set of aims related to nutrition/environment interactions. In these aims, we will characterize the influence of As exposure on histone modifications, relate changes in histone modifications to changes in DNA methylation, and characterize the impact of FA supplementation on these marks. Finally, using the Intinium Human Methylation450 array, we will identify a set of genes that are differentially methylated by As exposure and determine gene-specific histone modifications at these loci. Collectively, these aims will begin to elucidate the molecular events that underlie the effects of As and folate on DNA methylation. The implications of identifying an influence of FA supplementation on histone modifications are considerable, as this represents a simple, low-cost, low-risk intervention as a potential therapeutic approach to reverse As-induced epigenetic dysregulation.